Widgets need help text.

@paulaLee,

Can you write up some descriptions for each widget.

Here are lists for all the widgets:

- http://django-widgy.readthedocs.org/en/latest/contrib/page-builder/index.html
- http://django-widgy.readthedocs.org/en/latest/contrib/form-builder/index.html

The audience for these descriptions is users of Widgy, not developers.